The invention relates to lighting systems for flat panel displays, and more particularly to lighting systems that emit polarized light to a liquid crystal micro-display.
Flat panel micro-displays are utilized in devices such as video cameras, hand-held computers, and wireless telephones. A common flat panel micro-display technology is the liquid crystal display (LCD) technology. As is known in the field of LCDs, an integral part of a LCD is the lighting system that provides light to the LCD pixels.
Because LCD displays are often utilized in battery operated portable devices, an important consideration in all of the lighting technologies is the conservation of energy and the efficient use of generated light. One technology for lighting a micro-display that utilizes a reflective LCD panel is disclosed in U.S. Pat. No. 6,005,720 issued to Watters et al. (hereinafter Watters). In Watters, light from an off-axis light source is projected through a first prism onto a reflective LCD panel and then through a second prism. Although the lighting system works well, light having the undesired polarization component is not utilized to illuminate the reflective LCD panel. In addition, the prisms required to direct and polarize the light add substantial thickness to the overall micro-display package.
Another technology for illuminating a transmissive micro-display involves an edge lit flat illuminator. Edge lit flat illumination involves injecting light along the edge of a flat transparent panel, such as glass or plastic slab, and causing as much of the light as possible to illuminate the surface of the transmissive LCD panel.
Some edge lit flat illumination techniques provide unpolarized light to a transmissive LCD panel. However, because transmissive LCD panels are designed to transmit only one polarization component of light, other polarization components of the unpolarized light are absorbed by the LCD panel. Absorption of the other polarization components of the unpolarized light by the LCD panel reduces the brightness level that can be achieved by the micro-display and causes unwanted heating of the reflective LCD panel.
Edge lit flat illumination techniques are known that provide polarized light to a transmissive LCD panel. FIG. 1 represents a flat panel micro-display system that includes a transmissive LCD panel 104 located above an edge lit flat panel illumination system 106. The edge lit flat panel illumination system includes a light source 108, a flat panel lightguide 110, a polarizing layer 112, and reflective layer 114. The polarizing layer transmits light of one polarization state and reflects light of the other polarization state. The light source injects unpolarized light into the flat panel lightguide and light that is incident on the polarizing layer is either passed through the polarizing layer or reflected within the lightguide depending on the polarization state of the incident light. Typically, half of the light that is incident on the polarizing layer has a polarization state that is passed through the polarizing layer as indicated by the dashed line 130 and half of the light that is incident on the polarizing layer has a polarization state that is reflected by the polarizing layer as indicated by the solid line 132. The linearly polarized light that passes through the polarizing layer is then reflected by the reflective layer 114. The reflected light then passes up through the lightguide and illuminates the transmissive LCD panel. Light passed by the transmissive LCD panel can be seen from the viewing direction.
The light that is initially reflected by the polarizing layer 112 continues to propagate through the lightguide 110 as shown by the solid line 132 in FIG. 1 and is not utilized to illuminate the LCD panel 104. Eventually, the unused light exits the opposite edge of the waveguide or is dissipated within the lightguide. Because half of the initially generated light is not passed by the polarization layer to illuminate the LCD panel, the energy efficiency and light use efficiency of the micro-display described with reference to FIG. 1 are low.
One known improvement to the edge lit flat panel illumination system of FIG. 1 involves adding a depolarizing reflector to the edge of the flat panel lightguide that is opposite the light source. Referring to FIG. 2, light having the unwanted polarization state is initially reflected by the polarization layer 212 and propagates through the flat panel lightguide 210 until the light is incident on the depolarizing reflector 216 that is located at the opposite edge face of the lightguide. The depolarizing reflector depolarizes the light having the unwanted polarization state thereby causing a portion of the reflected light to change to the desired polarization state. The portion of the reflected light that is changed to the desired polarization state is then passed by the polarization layer as shown by dashed line 234 and reflected by the reflector 214 to illuminate the LCD panel 204. While this technique works well to improve the efficiency of the micro-display, there is still a substantial portion of light that does not change polarization state to the desired polarization state and therefore is not utilized to illuminate the reflective LCD panel.
In view of the need for more efficient lighting systems for micro-displays and in view of the prior art limitations, what is needed is a lighting system and method with improved efficiency.
An illumination system and method involve utilizing a polarization rotator and a reflector with a flat panel edge lit waveguide to rotate the polarization state of light that is initially reflected from a polarizing layer. The polarization rotator changes the polarization state of the initially reflected light from an undesired polarization state to a desired polarization state and the reflector reflects the portion of light back to the polarization layer where it is passed to illuminate a LCD panel. The combination of the polarization rotator and the reflector provide a controlled system in which most of the initially generated light is utilized to illuminate the LCD panel.
An embodiment of an illumination system includes a lightguide, a polarization system, and polarization/reflector combination that is integrated with the lightguide. In the embodiment, the lightguide has a first major surface opposite a second major surface and a first edge face opposite a second edge face, with the first edge face being positioned to receive light from a light source. The polarizing system, which is proximate to the second major surface of the lightguide, passes a first portion of the light having a desired polarization state to a display panel and reflects a second portion of said light having an undesired polarization state. The polarization rotator/reflector combination reflects the second portion of the light at the second edge face and rotates the polarization state of the second portion of light from the undesired polarization state to the desired polarization state, wherein the second portion of the light having the desired polarization state is incident on the polarizing system and is passed to the display panel.
A method for illuminating a display device involves inputting light into a first edge face of a lightguide, with the lightguide having a first major surface opposite a second major surface, passing a first portion of the light having a desired polarization state through a polarization system that is proximate to the first major surface of the lightguide, reflecting a second portion of the light having an undesired polarization state from the polarization system that is proximate to the first major surface of the waveguide, passing the second portion of the light through a polarization rotator to rotate the polarization state of the second portion of the light to the desired polarization state from the undesired polarization state, reflecting the second portion of the light at a second edge face that is opposite the first edge face, and passing the second portion of the light through the polarization system after the polarization state of the second portion of light has been rotated to the desired polarization state and after the second portion of the light has been reflected at the second edge face.
An advantage of the illumination system and method is that all, or nearly all, of the light that is initially generated from a light source is utilized to illuminate the LCD panel, thereby increasing the light and energy efficiency of the illumination system. Another advantage of the illumination system and method is that a LCD panel can be illuminated with a flat panel lightguide, which allows the thickness of a micro-display package to be thinner than off-axis micro-displays that utilize prism systems to illuminate a LCD panel.